The present invention relates to frequency signalling systems and, more particularly, to tone frequency signalling systems wherein information is tone encoded for transmission to a remote location where it is tone decoded and used as control or signalling information.
Over the years, a broad range of frequencies and signal formats have been developed and are now used in present day tone frequency signalling systems. This is particularly true in the area of two-way radio and car telephone systems, as independent manufacturers have separately developed their own tone frequency signalling systems using tone frequencies and signalling formats unique to their systems. Many tone frequency signalling systems employ contactless resonant reeds, or active filters or other electronic components to accomplish the required tone frequency encoding/decoding functions. The wide range of frequencies used in the various systems has required the maintenance by manufacturers and suppliers of a large stocked inventory of standard frequency components. Moreover, the various timing requirements employed in the different signalling schemes has required the stocking of additional numerous diverse timing components.
Accordingly, a principal object of the present invention is to provide a universal frequency encoder/decoder circuit which can be inventoried and factory programmed to generate and detect the frequencies used in present day and future frequency signalling systems, thus reducing the number of frequency components which must be stocked.
Another object of the invention is to provide a universal encoder/decoder circuit which can be used alone or with other like encoder/decoder circuits and external timing and memory circuits to form a tone frequency signalling system which can be suitably signal format and frequency programmed as desired.
A still further object of the invention is to provide a universal encoder/decoder circuit which is designed around digital techniques and can be simply packaged as a large scale integrated (LSI) circuit thereby minimizing the number of parts needed to construct a desired tone frequency signalling system.
Since many selective signalling systems are used in mobile communications systems, e.g. a car telephone environment, an additional object of the invention is to provide a universal encoder/decoder circuit which has minimized power consumption and which is capable of minimizing the power consumption of external timing and memory components connected therewith.
Frequency synthesizers are often employed to provide a selected one of a multitude of output frequencies. Usually, a reference clock source is frequency divided in a programmable divider by different integer divider factors to produce a desired output frequency. Very often the exact output frequency required cannot be obtained because of the limitations of the frequency of the clock source and the integer dividing nature of the programmable divider. Accordingly, a yet further object of the invention is to provide a unique frequency encoder which more accurately generates an output signal at a selected frequency.
Many present day tone frequency signalling systems have signalling formats requiring precise timing of tones and of gaps between tone bursts. Heretofore, discrete timing circuits have been devised for each different signalling format which again requires the stocking of a large number of parts. Accordingly, yet another object of the invention is to provide a universal programmable timing circuit which is particularly useful with one or more of the universal encoder/decoder circuits of the invention or with other encoder/decoder circuits to create the timing patterns of present and future tone frequency signalling systems.
These and other objects and advantages of the invention can be seen from the following detailed description thereof provided in accordance with the accompanying drawings.